The present invention relates to an optical transmitter for transducing an electrical signal into an optical signal for transmission, and more particularly to an optical transmitter for stabilizing transition times (rising time and falling time) of an optical signal against a varying operating temperature. The present invention is also related to an optical transmission system using this optical transmitter.
An example of conventional optical transmitters is shown, for example, in JP-A-2-215239 (hereinafter called the xe2x80x9cprior art (1)xe2x80x9d). FIG. 1 illustrates the configuration of this optical transmitter.
The illustrated optical transmitter is composed of an amplifier 4, a modulator 2, a current source 3, and a light emitting element 1. Data signals of positive phase and negative phase are inputted to terminals N1, N2, respectively. The data signals are amplified by the amplifier 4 which is composed of transistors Q3-Q8, a current source I1, and resistors R1-R4, and inputted to bases of a pair of differential transistors Q1, Q2 in the modulator 2. The modulator 2 controls to conduct and break a driving current generated by the current source 3 in response to the signals inputted to bases of the transistors Q1, Q2. As a result, a modulated current signal is outputted to the light emitting device 1 connected to a collector of the transistor Q1, causing the light emitting device 1 to generate an optical signal.
Another example of conventional optical transmitters is shown, for example, in JP-A-10-229232 (hereinafter called the xe2x80x9cprior art (2)xe2x80x9d). FIG. 2 illustrates the configuration of this optical transmitter. The illustrated optical transmitter is composed of a pair of differential transistors 101 for supplying a semiconductor laser diode 113 with a modulated driving current; a transistor 104 for supplying the semiconductor laser diode or laser diode 113 with a bias current; and emitter follower transistors 102, 103 for driving the differential transistor pair 101, wherein a pulsed driving current for alternately driving the semiconductor laser diode 113 is controlled by an automatic power control (APC) voltage. More specifically, the APC voltage is used to control a current flowing through the emitter follower transistors 102, 103 for driving the differential transistor pair 101, so that even if the pulsed driving signal to the differential transistor pair 101 varies to cause fluctuations in the speed of the differential transistor pair 101, the speed of the emitter follower transistors 102, 103 can be changed to cancel a fluctuating portion of the speed of the differential transistor pair 101.
In addition, it has been known that as a base-to-collector voltage of a transistor forming part of a modulator in an optical transmitter changes due to a varying temperature, a parasitic capacitance between the base and the collector of the transistor also changes, thereby resulting in deformation of the optical signal waveform. To solve this problem, JP-A-9-83456 (hereinafter called the xe2x80x9cprior art (3)xe2x80x9d) describes a technique for controlling a base voltage of the transistor in accordance with the temperature to compensate for a temperature dependency of the base-to-collector voltage of the transistor.
The optical transmitter according to the prior art (1), however, implies a problem that the optical signal rises and falls at different times depending upon the operating temperature. This problem results from the temperature characteristic of a bipolar transistor or a field effect transistor which forms part of the differential transistor pair of the modulator.
The following equations (1), (2) expresses the input/output characteristics (modulated current versus differential input voltage characteristics) of a modulator composed of bipolar transistors and a modulator composed of field effect transistors:                     Im        =                  Is                      1            +                          exp              ⁡                              (                                                                            q                      ·                      Δ                                        ⁢                                          xe2x80x83                                        ⁢                    V                                                                              K                      B                                        ·                    T                                                  )                                                                        (        1        )                                          Im          =                                    1              2                        ⁢                          (                              Is                +                                                                                                    q                        ·                        D                                                                                              k                          B                                                ·                        T                                                              ·                                          W                      L                                        ·                    Co                    ·                    Δ                                    ⁢                                      xe2x80x83                                    ⁢                                      V                    ·                                                                  (                                                                              4                            ⁢                            Is                                                                                                                                              q                                ·                                D                                                                                                                              k                                  B                                                                ·                                T                                                                                      ·                                                          W                              L                                                        ·                            Co                                                                          )                                                                                            -                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                                      V                    2                                                              )                                      ⁢                  
                ⁢        where                            (        2        )                                          Δ          ⁢                      xe2x80x83                    ⁢          V                 less than                               Is                                                            q                  ·                  D                                                                      k                    B                                    ·                  T                                            ·                              W                L                            ·              Co                                                          (        3        )            
Im is a modulated current; xcex94V, a differential input voltage; Is, a current source current; q, a charge; kB, the Boltzmann""s factor; T, an absolute temperature; W, a gate width; L, a gate length; Co, a gate capacitance per unit area; and D, a diffusion constant.
The equations (1), (2) respectively include a term xe2x80x9cxcex94V/T.xe2x80x9d It can be seen that the input/output characteristic of the modulator varies depending on the operating temperature.
FIGS. 3A-3D are diagrams for explaining the temperature dependency of the waveform of a modulated current signal which is generated using a modulation control signal in a conventional optical transmitter. A modulator illustrated in FIG. 3A exhibits a reduced slope (a changing rate of the modulated current with respect to a change in a differential input voltage) of the input/output characteristic (a modulated current Im with respect to a differential input voltage: xcex94V=V1xe2x88x92V2; where V1, V2 are base voltages of transistors Q1, Q2) when the operating temperature rises from T1 K to T2 K, as illustrated in FIG. 3C. The reduced slope of the input/output characteristic in turn results in an extended linear input range for the differential input voltage.
The optical transmitter according to the prior art (1) supplies the modulator having the temperature characteristic as mentioned with an input signal of a constant voltage amplitude as illustrated in FIG. 3B irrespective of the operating temperature. On the other hand, rising/falling times of the modulated current signal Im is determined by a transition time of the modulation control signal in a linear input voltage range. Due to the temperature dependency of the modulated current signal Im, if the operating temperature changes from T1 to T2 to cause a change in the slope of the input/output characteristic, the rising/falling times of the modulated current signal change as illustrated in FIG. 3D, where a solid line and a dotted line represent the modulated current signal at temperatures T1 and T2, respectively. Therefore, an optical signal generated from the modulated current signal will have a temperature dependency in rising/falling times.
Further, a laser diode and a light emitting diode (LED), which are light emitting elements, have their input/output characteristics changing depending on the temperature. FIG. 4A is a graph illustrating the input/output characteristic of a laser diode. As illustrated in FIG. 4A, as an operating temperature increases from T1 to T2 and further to T3 (T1 less than T2 less than T3) in the laser diode, a threshold value for a driving current ILD for the laser diode to start emitting light also increases from Ith1 to Ith2 and Ith3. Also, as the operating temperature increases from T1 to T2 and further to T3, a slope efficiency (xcex94Po/xcex94ILD), which is the slope of the input/output characteristic (driving current ILD versus output light amount Po), decreases as indicated by a solid line, a dotted line and a one-dot chain line.
Therefore, assuming that a collector current IQ1 of a transistor Q1, which serves as a driving current for a laser diode as illustrated in FIG. 4B, is constant irrespective of the operating temperature, the output light amount Po of the laser diode will decreases as the temperature rises, as illustrated in FIG. 4C. In FIG. 4C, a waveform drawn by a solid line indicates the output light amount Po when the operating temperature is at T1, and a waveform drawn by a dotted line indicates the output amount Po when the operating temperature is at T2.
Similarly, when a light emitting diode is employed as a light emitting element, a slope efficiency (xcex94Po/xcex94ILED), which is the slope of the input/output characteristic (driving current ILED versus output light amount Po), decreases as indicated by a solid line, a dotted line and a one-dot chain line as the operating temperature rises from T1 to T2 and T3, as illustrated in FIG. 5. Therefore, if the collector current IQ1 of the transistor Q1, which serves as a driving current for the light emitting diode, is constant irrespective of the operating temperature, the output light amount Po of the light emitting diode decreases as the operating temperature rises.
None of the prior arts (1), (2), (3) take into account a compensation for a change in the characteristic of the differential transistor pair due to the varying operating temperature or a compensation for a change in the characteristic of the light emitting element (laser diode) due to the varying operating temperature.
As described above, since the optical signal rises/falls at varying times depending on the operating temperature, the conventional optical transmitters have difficulties in generating a stable optical signal waveform over a wide range of operating temperature.
It is therefore an object of the present invention to provide an optical transmitter and an optical transmission system which eliminate the above-mentioned problems inherent to the prior art.
To achieve the above object, in one aspect of the present invention, there is provided an optical transmitter which comprises a modulator for generating a modulated current in accordance with a modulation control signal, a light emitting element driven by the modulated current from the modulator to emit light in accordance with the modulated current, a first current source for supplying the modulator with a driving current, a first temperature detector for detecting an operating temperature of the modulator to output a signal indicative of a detected operating temperature of the modulator, and an amplifier for receiving a data signal to supply the modulator with the modulation control signal based on the data signal, wherein the amplifier has a first modulation control signal controller for controlling a changing amount per unit time of the modulation control signal at rising and falling times in accordance with an output signal of said temperature detector.
In the optical transmitter configured as described above according to the present invention, the amplifier controls the slew rate of the modulation control signal to compensate for a temperature dependency of the input/output characteristic of the modulator. This result in substantially a constant transition time of the modulation control signal in a linear input voltage range of the modulator, thereby suppressing a temperature dependency of rising/falling times of the modulation control signal. Thus, an optical signal generated from the modulated current signal also has rising/falling times maintained substantially constant against the varying operating temperature.
Accordingly, the present invention can suppress the temperature dependency of the rising/falling times of the optical signal, thereby making it possible to provide an optical transmitter which exhibits a stable transition time over a wide range of temperature.
According to an example of the present invention, the optical transmitter further comprises a second temperature detector for detecting an operating temperature of the light emitting element to output a signal indicative of a detected operating temperature of the light emitting element, wherein the first current source controls the amount of driving current supplied to the modulator in accordance with an output signal of the second temperature detector. In this way, an output current of the first current source, which is a variable current source, is controlled in accordance with the operating temperature of the light emitting element, thereby compensating for a temperature dependency of a slope efficiency which is the slope of the input/output characteristic (driving current versus output light amount) of the light emitting element.